The present disclosure relates to semiconductor devices, particularly to nitride semiconductor devices which can be used as power transistors etc.
Nitride semiconductors represented by gallium nitride (GaN) are wide gap semiconductors. For example, GaN and AlN have band gaps as large as of 3.4 eV and 6.2 eV, respectively, at room temperature. The nitride semiconductors have high breakdown field strength, and high saturated drift velocity of electrons as compared with compound semiconductors such as gallium arsenide (GaAs) etc., or silicon (Si) semiconductors, etc. In a heterostructure of AlGaN and GaN on plane (0001), charges are generated at a heterointerface due to spontaneous polarization and piezoelectric polarization. The charges generated at the heterointerface between AlGaN and GaN have a sheet carrier concentration of 1×1013 cm−2 or higher even when AlGaN and GaN are undoped. A hetero-junction field effect transistor (HFET) having high current density, and low on-resistance can be provided by using two-dimensional electron gas (2DEG) generated at the heterointerface (see, for example, 1W. Saito et al., IEEE Transactions on Electron Devices, 2003, vol. 50, No. 12, p. 2528).
In a HFET using a heterostructure of AlGaN and GaN, characteristics of a FET can be improved by providing a p-type nitride semiconductor layer below a gate electrode (see, for example, Japanese Patent Publication No. 2006-339561). With the p-type layer provided below the gate electrode, pn junction is formed between the 2DEG, which is generated at an interface between the AlGaN layer and the GaN layer, and the p-type layer. Thus, gate leakage current is less likely to flow even when gate voltage is increased, and large drain current can be obtained. Further, a normally-off HFET can be provided.